Electrolyte rebalancing system

ABSTRACT

A process for rebalancing the electrolyte system in a regenerative fuel cell using a sulfide/polysulfide reaction in one half of the cell and a bromine/bromide reaction in the other half of the cell comprises passing the electrolyte containing sulfide/polysulfide or bromine/bromide through the + ve  chamber of an auxiliary cell and passing an electrolyte containing water and being free from polysulfide or bromine through the − ve  chamber of the auxiliary cell the auxiliary cell operating so as to oxidize sulfide ions to sulfur or bromide ions to bromine in the + ve  chamber and to reduce water to hydrogen and hydroxide ions in the − ve  chamber.

The present invention relates to the field of regenerative fuel cell(RFC) technology. In particular it relates to apparatus and methods forthe operation of RFCs which enhance their performance characteristics.

The manner in which RFCs are able to store and deliver electricity iswell known to those skilled in the art. An example of an RFC isdescribed in U.S. Pat No. 4,485,154 which discloses an electricallychargeable, anionically active, reduction-oxidation system using asulfide/polysulfide reaction in one half of the cell and aniodine/iodide, chlorine/chloride or bromine/bromide reaction in theother half of the cell. The two halves of the cell are separated by acation exchange membrane.

The overall chemical reaction involved, for example, for thebromine/bromide-sulfide/polysulfide system is shown in Equation 1 below:Br₂+S²⁻⇄2Br⁻+S  Equation 1

However, within an RFC such as that described in U.S. Pat. No.4,485,154, the reaction takes place in separate but dependent bromineand sulfur half-cell reactions as shown below in Equations 2 and 3:Br₂+2e ⁻⇄2Br⁻  Equation 2S²⁻⇄2e ⁻+S  Equation 3

It should be noted however that these equations represent the overallreactive changes occurring in the RFC. In practice the reactions arecomplicated by the low basicity of sulfide which results in theformation of bisulfide as the active species, as shown in Equation 4.S²⁻+H₂O⇄HS⁻+OH⁻  Equation 4

Also, the sulfur produced in Equations 1 and 3 forms soluble polysulfidespecies in the presence of sulfide ions, as shown in Equation 5 (where xmay be from 1 to 4).S²⁻ +xS⇄S_(x+1) ²⁻  Equation 5

Also, free bromine is solubilised in the presence of bromide ions toform the tribromide ion, as shown in Equation 6Br⁻+Br₂⇄Br₃ ⁻  Equation 6

When the RFC is discharging, bromine is converted to bromide on the+^(ve) side of the membrane and sulfide is converted to polysulfide onthe −^(ve) side of the membrane. Equation 1 goes from left to right andmetal ions flow from the −^(ve) side of the membrane to the +^(ve) sideof the membrane to complete the circuit. When the RFC is charging,bromide is converted to bromine on the +^(ve) side of the membrane andpolysulfide is converted to sulfide on the −^(ve) side of the membrane.Equation 1 goes from right to left and metal ions flow from the +^(ve)side of the membrane to the −^(ve) side of the membrane to complete thecircuit.

The discharge/charge cycle described above will be repeated many timesduring the lifetime of the RFC and in order for the RFC to workefficiently throughout its lifetime it is important that theelectrolytes remain balanced. In the context of the presentspecification, when the term “balanced” is used to describe theelectrolytes it means that the relative concentrations of the reactivespecies within the electrolytes are maintained at, or close to, valueswhich enable optimum performance of the RFC. Similarly, in the contextof the present specification, the term “rebalancing” refers to a processwhich alters the concentration of one or more reactive species in one orboth of the electrolytes so as to return said electrolytes to a balancedstate or so as to maintain said electrolytes in a balanced state.

At the beginning of the RFC's lifetime the relative concentrations ofthe reactive species on either side of the membrane will normally befixed so that the electrolytes are balanced. However, once the RFCbegins to operate in its repeating discharge-charge cycle, factors mayintervene which result in the electrolytes becoming unbalanced. Thesefactors will vary depending upon the identity of the reactive specieswithin the electrolytes and on the manner in which the RFC isconstructed and operated.

In the case of the bromine/bromide-sulfide/polysulfide RFC such as thatdescribed above, the most important factor which results in theelectrolytes becoming unbalanced is the diffusion of unwanted speciesacross the membrane. Although a cation selective ion-exchange membraneis used, 100% permselectivity is not possible and during extendedcycling of the cell some anionic species diffuse through the membrane.In particular, sulfide ions (largely present in the bisulfide form, HS⁻)and polysulfide ions (S_(x+1) ²⁻, where x may be from 1 to 4) maydiffuse from the sulfide/polysulfide electrolyte into thebromine/bromide electrolyte where they will be oxidised by the bromineto form sulfate ions as shown in equations 7 and 8 below:HS⁻+4Br₂+4H₂O→8Br⁻+SO₄ ²⁻+9H⁺  Equation 7S_(x+1) ²⁻+(3x+4)Br₂+(4x+4)H₂O→(6x+8)Br⁻+(x+1)SO₄ ²⁻+(8x+8)H⁺  Equation8

Imperfections other than diffusion through the membrane which couldsimilarly contribute to the above process are ineffective sealingbetween cell compartments, or catastrophic failure of any of the cellseparating components, each of which may result in crossover of theelectrolytes between cell compartments.

In Equations 7 and 8, the oxidation of the sulfur species goes beyondthat which occurs during normal operation of the RFC. That is to say,the sulfide and polysulfide ions are oxidised all the way to sulfateions. Consequently, in the case of sulfide ion cross-over (Equation 7),four bromine molecules per sulfide ion are consumed rather than thenormal one bromine molecule per sulfide ion which is consumed in thereaction scheme of Equation 1. Similar overconsumption of bromineresults from polysulfide cross-over (Equation 8) although to a slightlylesser extent. As a result, the bromine/bromide electrolyte becomesdischarged to a greater extent than the sulfide/polysulfide electrolyte.Thus, when the cell is discharging there is insufficient bromine presentto react with all the sulfide ions present thereby preventing completionof the discharge cycle. As a result, the voltage generated by the cellbegins to decline earlier in the discharge cycle than when theelectrolytes are balanced. in effect, the reactions represented byEquations 7 and 8 result in the conversion of some of the polysulfideions to sulfide because not all of the polysulfide ions are recovered ondischarge. Subsequent cycles repeat this process, further reducing thenumber of polysulfide ions present. Ultimately, there will beinsufficient polysulfide ions present to accept charge during the chargecycle. Since the electrochemistry has to continue if charging ismaintained, the next most favourable reaction occurs, i.e. water isreduced and the electrode on the −^(ve) side of the cell starts to gashydrogen.

It would therefore be advantageous to provide a process for rebalancingthe electrolytes in order to compensate for the unbalancing effect ofthe cross-over of sulfide and/or polysulfide electrolyte species intothe bromine electrolyte. Although it would be possible to replace theelectrolytes in the system with fresh electrolytes at periodicintervals, this is disadvantageous because of the economic implicationsand because of the environmental implications of the great amounts ofwaste electrolytes which would require to be disposed of.

Accordingly, the present invention provides an electrochemical processfor energy storage and/or power delivery comprising:

-   -   (i) maintaining and circulating electrolyte flows in a fully        liquid system in which the active constituents are fully soluble        in a single cell or in an array of repeating cell structures,        each cell with a positive (+^(ve)) chamber containing an inert        +^(ve) electrode and a negative (−^(ve)) chamber containing an        inert −^(ve) electrode, the chambers being separated from one        another by a cation exchange membrane, the electrolyte        circulating in the ^(−ve) chamber of each cell during power        delivery containing a sulfide (electrolyte 1, and the        electrolyte circulating in the +^(ve) chamber during power        delivery containing bromine (electrolyte 2),    -   (ii) restoring or replenishing the electrolytes in the +^(ve)        and −^(ve) chambers by circulating the electrolyte from each        chamber to storage means comprising a volume of electrolyte        greater than the cell volume for extended delivery of power over        a longer discharge cycle than the cell volume alone would        permit, and    -   (iii) rebalancing the electrolytes by circulating a fraction of        electrolyte 1 or electrolyte 2 through the +^(ve) chamber of an        auxiliary cell, said auxiliary cell comprising a +^(ve) chamber        containing an inert +^(ve) electrode and a −^(ve) chamber        containing an inert he electrode, the chambers being separated        from one another by a cation exchange membrane, the electrolyte        circulating through the −^(ve) chamber of the auxiliary cell        containing water and being free from polysulfide and free from        bromine during rebalancing, the auxiliary cell operating so as        to oxidise sulfide ions to sulfur or bromide ions to bromine in        the +^(ve) chamber and so as to reduce water to hydrogen and        hydroxide ions in the −^(ve) chamber.

The oxidation of bromide to bromine rebalances the electrolytes byrestoring the bromine which is reduced by reaction with migratingsulfide ions. Oxidation of bromide to bromine may also be thought of ascharging the bromine/bromide electrolyte since the chemical content ofthe bromine/bromide electrolyte changes in the same manner as when theRFC is in its charging cycle.

The oxidation of sulfide to sulfur rebalances the electrolytes byoxidising the equivalent amount of sulfide which would ordinarily havebeen oxidised by the halogen which was reduced by reaction withmigrating sulfide ions. Oxidation of polysulfide to sulfur may also bethought of as discharging the sulfide/polysulfide electrolyte since thechemical content of the sulfide/polysulfide electrolyte changes in thesame manner as in the RFC when it is in its discharging cycle.

In order that rebalancing of the electrolytes may occur, it is essentialthat during the rebalancing process the electrolyte circulating throughthe −^(ve) chamber of the auxiliary cell should be free from polysulfideand free from bromine. The reason for this is that these chemicalspecies are more readily reduced than water. If electrolyte 1 iscirculated through the +^(ve) chamber of the auxiliary cell and theelectrolyte circulating through the −^(ve) chamber of the auxiliary cellcontains polysulfide, then the reaction which will occur in the −^(ve)chamber will be reduction of polysulfide to sulfide rather thanreduction of water to hydrogen and hydroxide ions. This would result inno net change in the oxidation state of the sulfur species present inthe system. If reduction of water is to occur in the presence ofpolysulfide the −^(ve) electrode in the −^(ve) chamber must be speciallyconstructed to starve it of polysulfide. Similarly, if electrolyte 2 iscirculated through the +^(ve) chamber of the auxiliary cell and theelectrolyte circulating through the −^(ve) chamber of the auxiliary cellcontains bromine, then the reaction which will occur in the −^(ve)chamber will be reduction of bromine to bromide rather than reduction ofwater to hydrogen and hydroxide ions. This would result in no net changein the oxidation state of the bromine species present in the system. Ifreduction of water is to occur in the presence of bromine the −^(ve)electrode in the −^(ve) chamber must be specially constructed to starveit of bromine. Inclusion of such specially constructed electrodes isclearly undesirable from an economic and system maintenance viewpoint.

The rebalancing process may be applied continuously to the RFC wherein asidestream of the bromine/bromide or sulfide/polysulfide electrolytedrawn from the mainstream is diverted through apparatus suitable forcarrying out the rebalancing process. The rebalancing process may alsobe applied as a batch process wherein the fraction of thebromine/bromide or sulfide/polysulfide electrolyte which is removed fromthe RFC is treated in separate apparatus suitable for carrying out therebalancing process before being returned to the RFC.

It will be understood by those skilled in the art that a number ofreduction half-cell reactions may be used to counter the oxidation ofthe halide or sulfide. However, in the present invention, the otherhalf-cell reaction under alkaline conditions involves the reduction ofwater to hydrogen and hydroxide ions according to the half-cell reactionshown in Equation 8 below:2H₂O+2e ⁻⇄H₂+2OH⁻  Equation 8

Thus the rebalancing process may be represented by the reactions shownin Equations 9 and 10 below:

 2Br⁻+2H₂O⇄Br₂+H₂+2OH⁻  Equation 9S²⁻+2H₂O⇄S+H₂+2OH⁻  Equation 10

Similarly in an acidic medium the half cell reaction comprises2H⁺+2e ⁻⇄H₂  Equation 11

It will be appreciated that, although the process of oxidising thebromine/bromide or sulfide/polysulfide electrolyte can be used torebalance the electrolytes, there is still a net loss of active sulfurspecies from the cell. This is because the sulfide and polysulfide ionswhich cross to the bromine electrolyte and are oxidised to sulfate ionsare not recovered. Thus, in a preferred embodiment of the presentinvention, the process additionally comprises adding elemental sulfur ora sulfide salt to the sulfide/polysulfide electrolyte in an amount suchas to restore the initial concentration of active sulfur species.

In carrying out the process of the present invention the electrolytecirculating through the −^(ve) a chamber of the auxiliary cell may bewater. In this instance the electrolyte will generally circulate in aclosed system and there will be no change of pH of the bromine/bromideor sulfide/polysulfide electrolyte.

In an alternative manner of carrying out the process of the presentinvention the electrolyte circulating through the −^(ve) chamber of theauxiliary cell is a fraction of electrolyte 1 or 2 which has been madefree of polysulfide or bromine by electrochemical reduction thereof.This may be achieved by recirculating electrolyte 1 or 2 through the−^(ve) chamber of the auxiliary cell until all of the polysulfide orbromine has been reduced. The electrolyte circulating through the −^(ve)chamber of the auxiliary cell may then be returned to the main stream ofelectrolyte 1 or 2.

Alternatively, the electrochemical reduction of polysulfide or brominewhich may be present in electrolyte 1 or 2 respectively occurs withinthe −^(ve) chamber of a second auxiliary cell which comprises a +^(ve)chamber containing an inert +^(ve) electrode and a −^(ve) chambercontaining an inert −^(ve) electrode, the chambers being separated fromone another by a cation exchange membrane, the electrolyte circulatingthrough the +^(ve) chamber being a fraction of electrolyte 1 orelectrolyte 2. This may be achieved by recirculating electrolyte 1 or 2through the −^(ve) chamber of the second auxiliary cell until all of thepolysulfide or bromine has been reduced. The electrolyte circulatingthrough the −^(ve) chamber of the auxiliary cell may then be returned tothe main stream of electrolyte 1 or 2.

Another reason why the reduction of any bromine which may be present inelectrolyte 2 is important is because, as described in WO-A-00/03448,carrying out the RFC process of the present invention results in theproduction of sulfate ions in the bromine/bromide electrolyte asdescribed above with reference to Equation 7. The removal of sulfateions from the electrolyte can only be carried out by the process asdescribed in WO-A-00/03448 in the absence of free bromine whichotherwise interferes with the process. Thus, in a preferred embodiment,the electrolyte circulating through the −^(ve) chamber of the auxiliarycell during rebalancing is a fraction of electrolyte 2 and that fractionis subsequently treated to remove sulfate ions contained therein.

In carrying out the process of the present invention elemental sulfurand/or a sulfide salt may be added to the sulfide/polysulfideelectrolyte in an amount sufficient to restore the initial concentrationof sulfur species.

The present invention also provides for the use, in a process for energystorage and/or power delivery comprising:

-   -   (i) maintaining and circulating electrolyte flows in a fully        liquid system in which the active constituents are fully soluble        in a single cell or in an array of repeating cell structures,        each cell with a positive (+^(ve)) chamber containing an inert        +^(ve) electrode and a negative (−^(ve)) chamber containing an        inert −^(ve) electrode, the chambers being separated from one        another by a cation exchange membrane, the electrolyte        circulating in the −^(ve) chamber of each cell during power        delivery containing a sulfide (electrolyte 1), and the        electrolyte circulating in the +^(ve) chamber during power        delivery containing bromine (electrolyte 2),    -   (ii) restoring or replenishing the electrolytes in the +^(ve)        and −^(ve) chambers by circulating the electrolyte from each        chamber to storage means comprising a volume of electrolyte        greater than the cell volume for extended delivery of power over        a longer discharge cycle than the cell volume alone would        permit,    -   of a process comprising:        -   circulating a fraction of electrolyte 1 or electrolyte 2            through the +^(ve) chamber of an auxiliary cell, said            auxiliary cell comprising a +^(ve) chamber containing an            inert +^(ve) electrode and a −^(ve) chamber containing an            inert −^(ve) electrode, the chambers being separated from            one another by a cation exchange membrane, the electrolyte            circulating through the −^(ve) chamber of the auxiliary cell            containing water and being free from polysulfide and free            from bromine during rebalancing, the auxiliary cell            operating so as to oxidise sulfide ions to polysulfide or            bromide ions to bromine in the +^(ve) chamber and so as to            reduce water to hydrogen and hydroxide ions in the −^(ve)            chamber,            for the purpose of rebalancing electrolytes 1 and 2.

The present invention also includes within its scope apparatus forcarrying out a process as described above comprising:

-   -   (i) a single cell or an array of repeating cell structures, each        cell comprising; a +^(ve) chamber containing an inert +^(ve)        electrode and a −^(ve) chamber containing an inert −^(ve)        electrode the chambers being separated from one another by an        ion exchange membrane, an electrolyte circulating in the −^(ve)        chamber of each cell which contains a sulfide during power        delivery (electrolyte 1), and an electrolyte circulating in the        +^(ve) chamber which contains bromine during power delivery        (electrolyte 2),    -   (ii) storage and circulation means for each electrolyte for        restoring or replenishing the electrolytes in the +^(ve) and        −^(ve) chambers,    -   (iii) means for rebalancing the electrolytes comprising an        auxiliary cell which comprises a +^(ve) chamber containing an        inert +^(ve) electrode and a −^(ve) chamber containing an inert        −^(ve) electrode the chambers being separated from one another        by a cation exchange membrane, means for circulating a fraction        of electrolyte 1 or 2 through the +^(ve) chamber of the        auxiliary cell, an electrolyte containing water and being free        from polysulfide and free from bromine during rebalancing and        means for circulating said electrolyte through the −^(ve)        chamber of the auxiliary cell.

The present invention will be further described with reference to theaccompanying drawings in which:

FIG. 1A is a schematic view of a basic electrochemicalreduction-oxidation cell in which a sulfide/polysulfide reaction iscarried out in one half of the cell and a bromine/bromide reaction iscarried out in the other half of the cell;

FIG. 1B is a diagram of cell arrays using the system of FIG. 1A;

FIG. 2 is a block diagram of a fluid flow system using the cell of FIG.1A;

FIG. 3 is a flow diagram of an apparatus for carrying out a preferredembodiment of the process of the present invention.

FIG. 4 is a flow diagram of an apparatus for carrying out a preferredembodiment of the process of the present invention.

FIG. 5 is a schematic diagram of an apparatus for carrying out a furtherpreferred embodiment of the process of the present invention, includingthe removal of sulfate.

FIG. 6 is a flow diagram of an apparatus for carrying out a preferredembodiment of the process of the present invention, including theremoval of sulfate.

FIG. 7 is a graph of voltage versus time for a selected number of cyclesof a RFC which does not incorporate a rebalancing process in accordancewith the present invention.

FIG. 8 is a graph of voltage versus time for a selected number of cyclesof a RFC which does incorporate a rebalancing process in accordance withthe present invention.

FIG. 1A shows a cell 10 with a positive (+^(ve)) electrode 12 and anegative (−^(ve)) electrode 14 and a cation exchange membrane 16 whichmay be formed from a fluorocarbon polymer with sulfonic acid functionalgroups to provide charge carriers. The membrane 16 acts to separate the+^(ve) and −^(ve) sides of the cell 10 and is selected to minimizemigration of bromine from the +^(ve) side to the −^(ve) side and tominimize migration of sulfide and polysulfide ions from the −^(ve) sideto the +^(ve) side. An aqueous solution 22 of NaBr is provided in achamber 22C formed between the +^(ve) electrode 12 and the membrane 16and an aqueous solution 24 of Na₂S_(x) (where x may be from 2 to 5) isprovided in a chamber 24C formed between the −^(ve) electrode 14 and themembrane 16. A K₂S_(x) solution, which is more soluble and moreexpensive than the Na₂S_(x) solutions, is used in another embodiment.

When the cell is in the discharged state, a solution of NaBr of up to6.0 molar concentration exists in the chamber 22C of the cell and asolution of Na₂S_(x) at 0.5 to 1.5 molar, exists in chamber 24C of thecell. Higher molarity is possible with K₂S_(x).

As the cell is charged, Na⁺ions are transported through the cationmembrane 16, as shown in FIG. 1A, from the +^(ve) to the −^(ve) side ofthe cell. Free bromine is produced via oxidation of the bromide ions atthe +^(ve) electrode and dissolves as a tribromide or pentabromide ion.Sulfur is reduced at the −^(ve) electrode and the pentasulfide,Na₂S_(x), salt eventually becomes the monosulfide as the chargingproceeds to completion. At the +^(ve) side the following reactionoccurs,2Br⁻⇄Br₂+2e ⁻and at the −^(ve) side the following reaction occurs,S+2e ⁻⇄S²⁻.

The membrane separates the two electrolytes and prevents bulk mixing andalso retards the migration of sulfide and polysulfide ions from the−^(ve) side to the +^(ve) side, and the migration of Br⁻ and Br₂ fromthe +^(ve) to the −^(ve) side. Diffusion of the sulfide and polysulfideions across the membrane results in the electrolytes becoming unbalancedas described earlier.

When providing power, the cell is discharging. During this action,reversible reactions occur at the two electrodes. At the +^(ve) sideelectrode 12, bromine is reduced to Br⁻, and at the −^(ve) electrode,the S²⁻ ion is oxidized to molecular S. The electrons produced at the−^(ve) electrode form the current through a load. The chemical reactionat the +^(ve) electrode produces 1.06 to 1.09 volts and the chemicalreaction at the be electrode produces 0.48 to 0.52 volts. The combinedchemical reactions produce an open circuit voltage of 1.54 to 1.61 voltsper cell.

The present system is an anionically active electrochemical system.Therefore, the cation which is associated with them essentially takes nopart in the energy producing process. Hence, a cation of “convenience”is chosen. Sodium or potassium are preferred choices. Sodium andpotassium, compounds are plentiful, they are inexpensive and have highwater solubilities. Lithium and ammonium salts are also possibilities,but at higher costs.

FIG. 1B shows an array 20 of multiple cells connected in electricalseries and fluid parallel. Multiple mid-electrodes 13 (each one having a+^(ve) electrode side 12A and −^(ve) electrode side 14A) and endelectrodes 12E (+^(ve)) and 14E (−^(ve)) are spaced out from each otherby membranes 16 and screen or mesh spacers (22D, 24D) in all the cellchambers 22C, 24C, (portions of two of which 22D, 24D are shown by wayof example) to form end cells C_(E1) and C_(E2) and an array of midcells C_(M) (typically 10-20; but note much smaller and much highernumbers of cells can be accommodated). The end electrodes 12E (+^(ve))and 14E (−^(ve)) have internal conductors 12F and 14F (typically copperscreens) encapsulated therein and leading to external terminals 12G, 14Gwhich are connected to external loads (e.g. to motor(s) via a controlcircuit (CONT), the motor(s) may be used to drive a vehicle) or powersources (e.g. utility power grid when used as a load-levelling device).

FIG. 2 shows a free flow system, a power generation/storage systemutilizing one or more of the batteries or cell array formats 20. Eachcell 20 receives electrolyte through pumps 26 and 28 for the NaBr andNa₂S_(x) solutions (22 and 24, respectively). The electrolytes 22 and 24are stored in containers 32 and 34. The tanks 32, 34 can be replacedwith freshly charged electrolyte by substituting tanks containing freshelectrolyte and/or refilling them from charged supply sources via lines32R, 34R with corresponding lines (not shown) provided for drainingspent (discharged) reagent. The electrolytes 22 and 24 are pumped fromtanks 32 and 34, respectively, into the respective chambers 22C and 24Cby means of pumps 26 and 28.

FIG. 3 shows a free flow system in which an array of cells 20 aresupplied with bromine/bromide and sulfide/polysulfide electrolyte fromstorage tanks 41 and 42 via lines 43 and 44. Bromine/bromide electrolytemay be removed from storage tank 41 via line 50 which transfers it to anexternal electrochemical cell 51 wherein the bromide is oxidised tobromine as a half-cell reaction in an electrochemical process. The otherhalf-cell reaction involves the reduction of water to hydrogen andhydroxide ions. An aqueous electrolyte is stored in tank 52 andtransported to the electrochemical cell 51 via line 53. The reducedelectrolyte is passed via line 54 to tank 55 where hydrogen gas which isgenerated by the electrochemical reaction may be vented from the system.The electrolyte returns via line 56 to storage tank 52.

FIG. 4 shows a particularly preferred variation of the free flow systemillustrated in FIG. 3. In this embodiment the aqueous electrolyte whichis reduced in the external electrochemical cell 51 is alsobromine/bromide electrolyte which has been removed from storage tank 41to tank 52 via line 57. In this case, the reduction reaction willinitially involve reduction of any residual bromine to bromide and willsubsequently involve reduction of water to hydrogen and hydroxide ions.The reduced electrolyte may be subsequently returned to the storage tank41 along line 58.

Referring to FIG. 5, a schematic flow diagram is shown of the manner inwhich the electrolyte may be treated. A storage tank 41 contains theaqueous bromine/bromide electrolyte which may be circulated around themain RFC system (not shown). The first treatment of a first stream ofthe electrolyte from tank 41 is to remove free bromine by treatment inan appropriate bromine reduction module 61 which comprises an auxiliarycell. The electrolyte is circulated via module 61 and an intermediatestorage tank 62 until the bromine present in the electrolyte is reducedto bromide. When the reduction is complete the electrolyte is passed tostorage tank 63. A second stream of the electrolyte from tank 41 ispassed to an electrochemical cell module 64 where the bromide isoxidised to bromine as a half-cell reaction in an electrochemicalprocess. The other half-cell reaction involves the reduction of water tohydrogen and hydroxyl ions using as the electrolyte for the hydroxyl ionproduction the electrolyte from storage tank 63. The stream ofelectrolyte passing through module 64 in which bromide has been oxidisedto bromide is returned to the storage tank 41. The stream of electrolyteused in the complementary half-cell reaction may be passed to a furthertank 65 where it is then subjected to a sulfate removal treatmentaccording to the teaching of WO-A-00/03448 in module 66. The stream ofelectrolyte from which the sulfate has been removed is then returned tothe original storage tank 41. Storage tank 63 is provided withappropriate means to vent hydrogen produced in the water reductionreaction to a hydrogen stack along line 67.

FIG. 6 shows an alternative system for use in accordance with thepresent invention which includes a sulfate crystallization unit asdescribed in WO-A-00/03448. In this system a portion of bromide/bromineelectrolyte contaminated with sulfate ions is drawn from the main system80 via line 81 and held in a receiving tank 82. This electrolyte is thencirculated via lines 83 and 84 through the −^(ve) chamber 85 of anauxiliary cell 86 until substantially all of the bromine present in theelectrolyte has been reduced to bromide ions. The voltage applied acrosscell 86 is limited to ensure that reduction of water does not occur.When the current density has run down (thus indicating that theconversion of bromine to bromide is substantially complete), the voltageapplied to the cell 86 is increased to a value sufficient to causereduction of the water present in the electrolyte so as to generate H₂gas and OH⁻ ions. The electrolyte circulating through the +^(ve) chamberof the external auxiliary cell is either sulfide/polysulfide orbromine/bromide electrolyte taken from the main system. The oxidation ofone or the other of these electrolytes rebalances the system. Theremoval of water from the electrolyte is advantageous because it furtherincreases the concentration of bromide ions thereby reducing the sulfatesolubility and increasing the yield of sulfate on crystallisation. A tap87 is provided to draw off H₂ gas. When sufficient rebalancing hasoccurred, the electrolyte circulating through the −^(ve) chamber 85 ispassed from the receiving tank 82 to the crystalliser 88 via line 89.This electrolyte is then passed via line 90 through a filter 91 toremove the sulfate crystals and then it may be returned to the mainsystem via line 92. The embodiment shown in FIG. 6 shows an electrolytebeing drawn from the main system via line 93, passing through the +^(ve)chamber 94 of the auxiliary cell and returning the main system via line95.

The present invention will now be further described by reference to thefollowing examples.

COMPARATIVE EXAMPLE 1

A regenerative fuel cell of the type described above havingsulfide/polysulfide and bromine/bromide electrolytes was set up. Thecell had the following specifications:

electrode material: polyethylene impregnated with activated carbonelectrode area: 2000 cm² current density: 80 mA/cm² electrolyte volume:91 per electrolyte cycle time: 6 hours (i.e. 3 hours charge and 3 hoursdischarge) flow rate: 1000 ml/min membrane material: Nafion 115 ™

The cell was operated over 18 cycles (108 hours) and the cell voltagewas monitored throughout this period. The results are shown in FIG. 7.It can be seen that after a limited number of cycles (about 7) the cellvoltage limits early on the discharge cycle due to the lack of bromine.This problem gets worse as the number of cycles increases. It can beclearly seen on the graph that after about 12 cycles (72 hours) the cellfails to maintain a good voltage performance over the whole of the 3hour discharge cycle.

EXAMPLE 1

An identical RFC to that used in Comparative Example 1 was set up. Thistime the electrolytes were continuously rebalanced by oxidation of asidestream of the bromide/bromine electrolyte drawn from the mainstream.Oxidation occurred in one half of an external electrochemical cellwherein the electrolyte undergoing reduction in the other half of thecell was dilute aqueous sodium hydroxide. The external electrochemicalcell used was an MP cell from Electrocell AB having the followingspecifications:

anode material: platinum cathode material: nickel electrode area: 100cm² current density: 13 mA/cm² flow rate: 270 ml/min membrane material:Nafion 350 ™

FIG.3 shows a schematic representation of the apparatus used in thepresent example. The cell was operated over at least 92 cycles (552hours) and the cell voltage was monitored throughout this period. Theresults from the period from 400 to 550 hours are shown in FIG. 8. Itcan be seen that even after 91 cycles (546 hours) the cell voltage doesnot limit early on the discharge cycle as occurred in the unbalancedcell. The cell retains a good voltage performance over the whole of the3 hour discharge cycle.

1. An electrochemical process for energy storage and/or power deliverycomprising: (i) maintaining and circulating electrolyte flows in a fullyliquid system in which the active constituents are fully soluble in asingle cell or in an array of repeating cell structures, each cell witha positive (+^(ve)) chamber containing an inert +^(ve) electrode and anegative (−^(ve)) chamber containing an inert −^(ve) electrode, thechambers being separated from one another by a cation exchange membrane,the electrolyte circulating in the −^(ve) chamber of each cell duringpower is delivery containing a sulfide (electrolyte 1), and theelectrolyte circulating in the +^(ve) chamber during power deliverycontaining bromine (electrolyte 2), (ii) restoring or replenishing theelectrolytes in the +^(ve) and −^(ve) chambers by circulating theelectrolyte from each chamber to storage means comprising a volume ofelectrolyte greater than the cell volume for extended delivery of powerover a longer discharge cycle than the cell volume alone would permit,and (iii) rebalancing the electrolytes by circulating a fraction ofelectrolyte 1 or electrolyte 2 through the +^(ve) chamber of antauxiliary cell, said auxiliary cell comprising a +^(ve) chambercontaining an inert +^(ve) electrode and a −ve chamber containing aninert −^(ve) electrode, the chambers being separated from one another bya cation exchange membrane, the electrolyte circulating through the−^(ve) chamber of the auxiliary cell containing water and being freefrom polysulfide and free from bromine during rebalancing, the auxiliarycell operating so as to oxidise sulfide ions to sulfur or bromide ionsto bromine in the +^(ve) chamber and so as to reduce water to hydrogenand hydroxide ions in the −^(ve) chamber.
 2. A process as claimed inclaim 1 wherein the electrolyte circulating through the −^(ve) chamberof the auxiliary cell during rebalancing is a fraction of electrolyte 1or electrolyte 2 which has been made free of polysulfide or bromine byelectrochemical reduction thereof.
 3. A process as claimed in claim 2wherein the electrochemical reduction of polysulfide or bromine iseffected by recirculating the fraction of electrolyte 1 or 2 through the−^(ve) chamber of auxiliary cell until all of the polysulfide or brominehas been reduced.
 4. A process as claimed in claim 2 wherein theelectrochemical reduction of polysulfide or bromine occurs within the−^(ve) chamber of a second auxiliary cell which comprises a +^(ve)chamber containing an inert +^(ve) electrode and a −^(ve) chambercontaining an inert −^(ve) electrode, the chambers being separated fromone another by a cation exchange membrane, the electrolyte circulatingthrough the +^(ve) chamber being a fraction of electrolyte 1 orelectrolyte
 2. 5. A process as claimed in claim 4 wherein theelectrochemical reduction of polysulfide or bromine is effected byrecirculating the fraction of electrolyte 1 or 2 through the −^(ve)chamber of the second auxiliary cell until all of the polysulfide orbromine has been reduced.
 6. A process as claimed in claim 3 wherein theelectrolyte circulating through the −^(ve) chamber of the auxiliary cellduring rebalancing is a fraction of electrolyte 2 and wherein thefraction 18 subsequently treated to remove sulfate ions containedtherein.
 7. A process as claimed in claim 6 wherein said sulfate ionsare removed by crystallisation of a sulfate salt from the fraction ofelectrolyte
 2. 8. A process as claimed in claim 2 wherein the fractionof electrolyte 1 or 2 which is circulated through the −^(ve) chamber ofthe auxiliary cell is returned to the main stream of electrolyte 1 or 2respectively.
 9. A process as claimed in claim 1 which additionallycomprises adding elemental sulfur and/or a sulfide salt to electrolyte 1in an amount sufficient to restore the initial concentration of sulfurspades.
 10. A process for rebalancing electrolyes in a process forenergy storage and/or power delivery comprising: (i) maintaining andcirculating electrolyte flows in a fully liquid system in which theactive constituents are fully soluble in a single cell or in an array ofrepeating cell structures, each cell with a positive (+^(ve)) chambercontaining an inert +^(ve) electrode and a negative (−^(ve))chambercontaining an inert −^(ve) electrode, the chambers being separated fromone another by a cation exchange membrane, the electrolyte circulatingin the −^(ve) chamber of each cell during power delivery containing asulfide (electrolyte 1), and the electrolyte circulating in the +^(ve)chamber during power delivery containing bromine (electrolyte 2), (ii)restoring or replenishing the electrolytes in the +^(ve) and −^(ve)chambers by circulating the electrolyte from each chamber to storagemeans comprising a volume of electrolyte greater than the cell volumefor extended delivery of power over a longer discharge cycle than thecell volume alone would permit, and (iii) circulating a fraction ofelectrolyte 1 or electrolyte 2 through the +^(ve) chamber of anauxiliary cell, said auxiliary cell comprising a +^(ve) chambercontaining an inert +^(ve) electrode and a −^(ve) chamber containing aninert −^(ve) electrode, the chambers being separated from one another bya cation exchange membrane, the electrolyte circulating through the−^(ve) chamber of the auxiliary cell containing water and being freefrom polysulfide and free from bromine during rebalancing, the auxiliarycell operating so as to oxidise sulfide ions to polysulfide or bromideions to bromine in the +^(ve) chamber and so as to reduce water tohydrogen and hydroxide ions in the −^(ve) chamber, for the purpose ofrebalancing electrolytes 1 and
 2. 11. An electrochemical apparatus forenergy storage and/or power delivery comprising: (i) a single cell or anarray of repeating cell structures, each cell comprising; a +^(ve)chamber containing an inert +^(ve) electrode and a −^(ve) chambercontaining an inert −^(ve) electrode the chambers being separated fromone another by an ion exchange membrane, an electrolyte circulating inthe −^(ve) chamber of each cell which contains a sulfide during powerdelivery (electrolyte 1), and an electrolyte circulating in the +^(ve)chamber which contains bromine during power delivery (electrolyte 2),(ii) storage and circulation means for each electrolyte for restoring orreplenishing the electrolytes in the +^(ve) and −^(ve) chambers, (iii)means for rebalancing the electrolytes comprising an auxiliary cellwhich comprises a +^(ve) chamber containing an inert +^(ve) electrodeand a −^(ve) chamber containing an inert −^(ve) electrode the chambersbeing separated from one another by cation exchange membrane, means forcirculating fraction of electrolyte 1 or 2 through the +^(ve) chamber ofthe auxiliary cell, an electrolyte containing water and being free frompolysulfide and free from bromine during rebalancing and means forcirculating said electrolyte through the −^(ve) chamber of the auxiliarycell.
 12. Apparatus as claimed in claim 11 wherein the means forcirculating an electrolyte through the −^(ve) chamber of the auxiliarycell comprises means for circulating a fraction of electrolyte 1 or 2through the −^(ve) chamber of the auxiliary cell.
 13. Apparatus asclaimed in claim 11 wherein the means for circulating an electrolytethrough the −^(ve) chamber of the auxiliary cell comprises a storagetank into which a fraction of electrolyte 1 or 2 may be transferred andmeans for re-circulating the fraction of electrolyte 1 or 2 between the−^(ve) chamber of the auxiliary cell and said storage tank. 14.Apparatus as claimed in claim 12 which additionally comprises a secondauxiliary cell which comprises a +^(ve) chamber containing an inert+^(ve) electrode and a −^(ve) chamber containing an inert −^(ve)electrode, the chambers being separated from one another by a cationexchange membrane, means for circulating a fraction of electrolyte 1 or2 through the +^(ve) chamber and means for circulating a fraction ofelectrolyte 1 or 2 through the −^(ve) chamber.
 15. Apparatus as claimedin claim 14 wherein the means for circulating an electrolyte through the−^(ve) chamber of the second auxiliary cell comprises a storage tankinto which a fraction of electrolyte 1 or 2 may be transferred and meansfor re-circulating the fraction of electrolyte 1 or 2 between the −^(ve)chamber of the second auxiliary cell and said storage tank. 16.Apparatus as claimed in claim 12 wherein the electrolyte circulatedthrough the −^(ve) chamber of the auxiliary cell is electrolyte 2,additionally comprising means for removing sulfate ions from thefraction of electrolyte 2 after circulation through the −^(ve) chamberof the auxiliary cell.
 17. Apparatus as claimed in claim 16 wherein themeans for removing sulfate ions from electrolyte 2 comprises acrystalliser.
 18. Apparatus as claimed in claim 11 additionallycomprising means for passing the fraction of electrolyte 1 or 2 which iscirculated through the −^(ve) chamber of the auxiliary cell back to themain stream of electrolyte 1 or 2 respectively.
 19. An apparatus forrebalancing electrolytes in an electrochemical apparatus for energystorage and/or power delivery comprising: (i) a single cell or an arrayof repeating cell structures, each cell comprising; a +^(ve) chambercontaining an inert +^(ve) electrode and a −^(ve) chamber containing aninert −^(ve) electrode the chambers being separated from one another byan ion exchange membrane, an electrolyte circulating in the −^(ve)chamber of each cell which contains a sulfide during power delivery(electrolyte 1), and an electrolyte circulating in the +^(ve) chamberwhich contains bromine during power delivery (electrolyte 2), and (ii)storage and circulation means for each electrolyte for restoring orreplenishing the electrolytes in the +^(ve) and −^(ve) chambers, and(iii) an auxiliary cell which comprises; a +^(ve) chamber containing aninert +^(ve) electrode and a −^(ve) chamber containing an inert −^(ve)electrode the chambers being separated from one another by a cationexchange membrane, means for passing a fraction of electrolyte 1 or 2through the +^(ve) of the auxiliary cell, an electrolyte containingwater and being free from polysulfide and free from bromine duringrebalancing and means for circulating said electrolyte through the−^(ve) chamber of the auxiliary cell for the purpose of rebalancingelectrolytes 1 and 2.